4|)J 56 SOME REMARKS ON THE AGENCY OF MANURES. 1(5.' 
SOME REMAEKS ON THE AGENCY OE MANURES.— HUMUS. 
By Mr. J. TOWEKS, C.M.H.S. 
THAT peculiar substance, now called humus, which has excited so much interest for the last seven- 
teen years, and given rise to so many wild and conflicting theories, ought, doubtless, to be con- 
sidered one of the most important constituents of every fertile soil. It is a product of vegetable and 
animal decay, and was once affectedly pronounced to be " the cooked food of plants." The reader may 
be referred back to the recondite article on the Chemistry of Soils, by Dr. Voelcker (ii., 39), where the 
compound nature of humus, as it generally exists, is amply described. I now propose to offer a few 
additional remarks, to elucidate all that appears to have been recently discovered. 
Dr. Voelcker tells us, " that humus is composed of a great many organic acids and products of 
vegetable decomposition," — this is true ; and, therefore, it is found abundantly in rich vegetable mould, 
and particularly in the black remains of an old heap, composed of tree leaves and spray combined with 
stable manure. The precise nature and quantity of those organic acids, though of little consequence 
to the cultivator, have become objects of interest to the analytic chemist ; thus it has been proved that 
two of them called crenic and apocrenic acid, are extracted by the simple process of boiling humus in 
pure distilled water ; a third, the humic acid, by treating the residue of the former process with 
boiling solution of carbonate of soda ; and a fourth by treating the insoluble humus which remains 
after the above-named processes, with solution of caustic potash. Other acids are named — such as the 
geic and ulmic — which are supposed to require specific and appropriate re-agents ; but the inquiry is 
devoid of all interest, otherwise than as it may throw some light upon the compound nature of those 
substances that are produced by the fermentation of animal and vegetable remains in every compost- 
heap. Admitting the inutility of minute researches, the great fact of the presence of humus in every 
fertile soil remains undisturbed ; and, I now allude to it particularly, with a view to excite attention 
to the results that invariably follow the application of lime, either in agriculture or gardening. 
If a portion of black reduced manure, or even of rich garden earth, glutted with dung, be boiled in 
rain water for a few minutes, a certain quantity of colouring matter will be extracted. On adding a 
small quantity of any of the alkalies to the mixture, and continuing the boiling for a few minutes, the 
colour will become much more intense, amounting, perhaps, to that of porter. The fluid so obtained 
we may style hamate of potash, soda, or ammonia, as the case may be. When separated from the 
dregs and left at rest to deposit its feculences, the clear liquid will deposit nearly the whole of the 
colouring matter if some clear lime-water be mixed with it. As a converse of this experiment, let a 
teaspoonful or two of powdered quick-lime be incorporated with an ounce or so of black spit dung, 
old decayed wood, or black peat, and the like humous matter ; you then may add a hot solution of any 
of the three named alkalies, and continue the boiling for any reasonable time without obtaining any 
intensity of colour. Lime, therefore, is proved to be the most powerful agent in attracting and fixing 
the acids ; or, in other words, those substances existing in vegetable mould, which are soluble in 
alkaline solutions. Hence, we find that pure lime (not chalk) is a specific remedy for soils rendered 
barren or sour by over-manuring ; and a meliorator in reclaiming morasses or peat-bogs. 
Liebig, in his zeal to establish the theory of vegetable nutrition by the absorbent faculty of the 
leaves, observes that "humus acts in the same manner in a soil permeable to air, as in the ah' itself; 
it is a continued source of carbonic acid, which it emits very slowly. An atmosphere of carbonic acid 
formed at the expense of the oxygen of the air, surrounds every particle of decaying humus ; it is, there- 
fore, contained in every fertile soil, and is the first and most important food for the young plants grow- 
ing upon it." — {Liebig s Chemistry, edit. 4, p. 30). " Humus is soluble only when combined with oxygen ; 
it can be taken up with water, therefore, only as carbonic acid. When moisture is absent, humus may 
be preserved for centuries ; but when moistened with water, it converts the surrounding oxygen into 
carbonic acid." — {Idem, p. 96). 
This great chemist strongly advocated a mineral theory, which Mr. Pusey has noticed in the Royal 
Agricultural Society's Journal (No. 26) : — " The mineral theory he eagerly adopted was contained in the 
following axiom of Liebig : — ' The crops in a field diminish or increase in exact proportion to the 
diminution or increase of the mineral substances conveyed to them in manure.' This doctrine received 
its death-blow from Mr. Lawes' experiments at Rothamstead in the following manner : — Wheat was 
grown on an arable field, exhausted for the purpose; unmanured,\t produced per acre 16| bushels of wheat; 
with 14 tons of farm-yard dung, 22 bushels; but with the ash of 14 tons of such dung 16 bushels only." 
The bare fact that soil unmanured yielded more wheat than when treated with all the ashes produced rj 
from 14 tons of that very manure which, when entire, gave an increase of 5i bushels, speaks volumes. 
H"tj\] At a future opportunity I hope to make this appear. 
